[exim] Exim 4.60 -error saving a message to a public folde…

Hello,
based on a .forward file I try to save a message to a public folder.
The test users .forward file looks like this:

# Exim filter

  if $header_subject: contains "*SPAM*" then
    seen
    save "/home/imappublic/Spam-Mail"
  endif

Every time a spam message arrives Exim tries to save it to the
file but without success. The following message is written to
mainlog:

mailbox /home/imappublic/Spam-Mail has wrong uid (2436 != 2006)

Now I wonder why this message appears although the user 'test',
with uid '2006', has write access to the file and why the delivery
process insists on the same uid.

The users:
  imappublic  uid: 2436  gid: 100
  test        uid: 2006  gid: 100

group:
  users       gid: 100

The directory and file permissioning:
  /home                       drwxr-xr-x  root        root
  /home/imappublic            drwxrwxrwx  imappublic  users
  /home/imappublic/Spam-Mail  -rw-rw-rw-  imappublic  users

Thank you in advance!
Juergen
--
Mail: juergen.edner@???
GPG Key available